The present invention relates to the general field of pedestrian barriers, and more particularly to the field of barriers used to control and direct groups of people in public places.
Queue barriers are commonly used to guide and control crowds of people at public events and exhibits. Typical freestanding queue barriers comprise a draped rope or retractable belt stretched between upright tubular stanchions, each mounted on a weighted circular base. For aesthetic reasons, it is often desirable to minimize the diameter of the stanchions and the bulk of the base. The preference for a sleek, unobtrusive look, particularly at artistic exhibits, can dictate the use of slender cords rather than belts between the stanchions.
While spring-loaded spool mechanisms are suitable for use with retractable belt barriers, a spool for the equivalent length of cord would need to be much wider—requiring an unsightly larger stanchion diameter. For retractable cord barriers, proper cord tension is a critical element, since a sagging cord is a visual distraction, while an excessively taut, unyielding cord can pose a tripping or safety hazard.
The present invention addresses these requirements by providing a retraction mechanism in which the cord is helically wound around one or more pairs of opposing pulleys. When the cord is extended, one set of pulleys in each pair remains fixed, while the other slides toward it against the resistance of a constant-force spring. In order to achieve the proper balance of cord and spring tension, the optimal stretch factor of the cord is less than 50%, as compared to 100% stretch cord commonly used in other applications. The optimal stretch factor of the cord is selected to achieve the correct balance between the retraction force of the spring, which is constant, and the extension force of the cord, which increases as the cord stretches. The excessive stiffness of 100% stretch cord translates into a large force that must be exerted to extend the cord. That large extension force must be balanced by an equally large refraction force of the spring, thereby requiring a large spring. But the refraction force of a large spring will cause a stanchion to tip over unless its base is heavily weighted. High spring tension will also cause an extended cord to snap back forcefully and hazardously when released. On the other hand, a cord with minimal or no stretch will be unyielding when taut and can become slack and develop an unsightly sag when extended between stanchions.
There are several U.S. patents directed to spring-biased retraction mechanisms. The systems described in the U.S. patents of Carlson (U.S. Pat. No. 5,117,859), Schwendinger (U.S. Pat. No. 6,338,450) and Bertagna et al. (U.S. Pat. No. 5,421,530) do not employ constant force springs, because there is no need in these applications to maintain a constant tension on the extended hose/cable/cord. Moreover, since the stretch factor of the hose/cable/cord in these applications is negligible, these mechanisms do not need to balance the opposing forces of a spring and a stretched cord, as does the present invention.
While the phone cord rewinder described in the U.S. patent of Ditzig (U.S. Pat. No. 5,507,446) does utilize a constant-force coiled metal spring as the biasing mechanism between the pulleys, it lacks any means of maintaining a constant taut tension on the extended phone cord, which must have a certain amount of slack to be usable.
The U.S. patent of Knapp et al. (U.S. Pat. No. 6,143,985) discloses a cable retracting system for modular components, using a pulley system biased by constant-force coiled metal springs. Unlike the Ditzig mechanism, this apparatus is designed to maintain a low constant force on the extended cable sufficient to prevent dangling and entanglement. But the Knapp system is incapable of providing the “straight line” tension required in a queue barrier and cannot be adapted to handle a stretchable cord.
In short, none of the spring-biased pulley retraction mechanisms disclosed in the prior art address the problem of achieving a constant taut, but yielding, tension in a stretchable cord. Nor can the features of the prior art mechanisms be combined in an obvious way to achieve this functionality of the present invention.